Recently, benefited from the drastic development of the epitaxy technology, LED semiconductor technology has successfully improved the light emitting efficiency of LED chips. As such, LEDs are more widely used, for example in projection pens, and illuminations. LEDs are known as having advantages of small sizes, longer lifetime, low contamination, and low cost. In addition, LEDs have better optical characteristics such as good color saturation, and ideal dynamic color control. In these manners, LED and related technologies are specifically concerned for further development.
Generally, LEDs have less power consumption, less pollution, and smaller sizes than conventional lamps (e.g. light bulbs). Besides, the appearance of alternating-current-driven (AC-driven) LED makes the LED illumination system has an advantageous position in the market. However, alike the direct-current-driven (DC-driven) LED chip, the AC-driven LED chip has the same problem of having a low light emitting efficiency. The current LED chips convert most of the inputted power into heat while few of the inputted power is converted into light. Therefore, how to avoid overheating is a major issue in the field of LED. Unfortunately, under the situation that the light emitting efficiencies of the current LED chips is limited, most of the inputted power is converted into heat causing a drastic increase on heating generating density. In such a way, the overheat problem is a bottleneck in technology development. Thermal resistance, is defined as a quantity of a junction temperature of an LED being subtracted the ambient temperature and further divided by a corresponding input power. The thermal resistance is a criterion for evaluating a capability of heat dissipation of an LED package. A larger thermal resistance indicates a worse capability of heat dissipation, while a smaller thermal resistance indicates a better capability of heat dissipation.
When packaging an LED, an upper limit of the thermal resistance should be restricted. As such, the measurement of thermal resistances of components is important and representative. As to the parameters for calculating the thermal resistance, the ambient temperature can be conveniently obtained by measuring with a thermal couple. As to the thermal-generating component, the inputted power is also known and easy to obtain. However, a junction of a packaged LED is encapsulated inside, and therefore the junction temperature of LED chip is difficult to measure with a direct measurement. As such, the junction temperature is usually measured with an indirect electrical measurement. If the method can fast and accurate measuring the thermal conductivity and the effect on optical characteristics due to heat of the component to be tested, it would be helpful in facilitating to the design and testing of the thermal conductivity of LED chips.
As same as the DC-driven LED chip, the junction temperature of the AC-driven LED chip could not be measured directly, such that the junction temperature of the AC-driven LED chip is also measured with an indirect electrical measurement. However, since the AC-driven LED chip is driven by an AC voltage rather than a DC voltage, the indirect electrical measurement for the thermal resistance of the AC-driven LED chip is different from that for the DC-driven LED chip. The AC voltage is a sine periodic wave, such that the voltage value thereof is not constant as the DC voltage. Establishment of the indirect electrical measurement for the thermal resistance of the AC-driven LED chip is helpful to improve the design of the packaged AC-driven LED chip.
Taiwan patent publication No. 200925571 disclosed an apparatus for measuring LED characteristic and chip temperature. The process of the measurement has two major steps. One step is to measure a bias voltage and a temperature sensitive parameter (TSP) by a thermal conductive type approach and an inputting-pulse type approach. Another step is to measure the voltage value of the LED chip under the real operation, then to calculate the temperature and the thermal resistance of the packaged LED chip according to the TSP correlation curve. However, the apparatus and method are only capable of measuring the DC-driven LED chip rather than the AC-driven LED chip. Moreover, the apparatus and method are limited to measure the thermal characteristic of the DC-driven LED chip, but the apparatus and method fails to integrate the methods for measuring the illumination characteristic, the thermal characteristic, and the electric characteristic of the DC-driven LED chip.
U.S. patent application Ser. No. 12/353,294 discloses a method for measuring the junction temperature of an AC-driven LED chip. The method for measuring the AC-driven chip is similar to the method for measuring the DC-driven chip. The AC-driven LED chip is measured by the way of inputting an AC voltage. The method of AC-driven measurement has two major steps. In the first step, an initial voltage is applied to the LED chip, the temperature of a substrate of the LED chip is controlled, and the current of the LED chip and the temperature of the substrate are measured, such that a characteristic curve of the LED chip could be obtained. In the second step, a resistor having a slight resistance is connected to the LED chip in series so as to really handle the input of the rated AD current, and a dual-channel data acquisition (DAQ) card is used to simultaneously extract the resistance value of the resistor and the voltage value of the AC-voltage. Then, according to the waveform of the applied AC voltage, the initial voltage of the LED chip and the corresponding voltage value of the resistor are found, so as to calculate the current value. Moreover, a current difference between the initial current and the heat-balanced current, so as to obtain the temperature difference of the LED chip according to the characteristic curve of the LED chip. Then, the junction temperature of the LED chip could be obtained by summing up the temperature difference and the initial temperature.
Although the method disclosed by Taiwan patent publication No. 200925571 could be used to speedily measure the thermal resistance of the LED chip, but the method is limited to measure the thermal characteristic of the DC-driven LED chip and fails to integrate the methods for measuring the illumination, thermal and the electric characteristics of the DC-driven LED chip. The method disclosed by patent application Ser. No. 12/353,294 could be used to measure the AC-driven LED chip, but it is too complicated and needs two steps while an extra resistor should be series-connected to the LED chip when the second step is performed. Since the result of the measurement is influenced by the resistance of the resistor, and it is hard to determine which resistor should be selected, the method is inconvenient and hard to control.